As discussed in a patent application Ser. No. 10/956643 entitled Method and Apparatus for Improved Determination of Range and Angle of Arrival Utilizing a Two-tone CW Radar by Paul D. Fiore, filed on even date herewith, assigned to the assignee hereof and incorporated herein by reference, a system is provided for providing range and angle of arrival estimates from the output of a two-tone CW radar. In this system, the range of an object from the radar is computed from the phase angle between returns from the object in which the phase of the Doppler return of one tone is compared with the phase of the Doppler return of the second tone.
This system uses a two-tone CW radar in which the two tones are sequentially projected or propagated towards a target. In one embodiment the switching rate between the two tones is on the order of 100 KHz, which corresponds to 5 milliseconds of the f1 tone followed by 5 milliseconds of the f2 tone.
When used for a fire control system to detect the range of a moving target, the system works relatively well for slow targets. However, when the target's speed approaches 300 meters per second, as in the case with rocket-propelled grenades, range estimates degrade significantly.
While initially a plurality of causes was investigated to ascertain the cause of the range error, it was noticed that the Doppler frequency associated with the 300 m/sec. target was about 49 KHz. This was found to be quite close to the 50 KHz Nyquist rate associated with the 100 KHz switching. The result with uncompensated systems was wide swings in the range estimate for incoming targets, whether the target was a rocket-propelled grenade, a projectile or a missile.
By way of background, the theory of two-tone continuous wave range estimation radar shows that target range is proportional to the difference in the complex phase angle between the signal returns corresponding to the two tones. In the above-mentioned sequential transmission of tones, known as a diplexing method, the two tones are transmitted sequentially, and it was assumed that the target Doppler frequency was small compared to the switching rate. With this assumption, an acceptably small bias in the range estimate results. However, it was found that the bias rate increases as the target speed increases, thus limiting the ability to accurately obtain the range of high-speed targets.
For a radar to measure range, it is typically thought that some sort of amplitude or phase modulation of the carrier is required. However, as mentioned above there is a method using more than one CW signal that can in fact provide range, which involves a tellurometer and is available for geodetic survey work. The geodetic system makes use of the fact that the survey equipment is not moving and therefore has a zero Doppler shift.
Radar designs for the case where there is target velocity and it is low can produce desired range estimates when using two-tone CW-transmitted signals. Additionally, approaching or receding targets can be distinguished through proper choice of CW frequencies.
Thus, those two-tone CW radars provide accurate range measurements if the motion during one Doppler period is small. This means that the phases of the wave forms will not appreciably “slip” relative to each other and a comparison between the phases of the wave forms can be made.